Steel

ABSTRACT

According to an aspect of the present invention, there is provided a steel containing, by mass %, C: 0.08% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.00% to 3.00%, P: 0.040% or less, S: 0.020% or less, Cr: 1.0% to 2.5%, Cu: 0.01% to 0.50%, Ni: 0.75% to 3.20%, Mo: 0.10% to 0.50%, Nb: 0.005% to 0.050%, Al: 0.010% to 0.100%, N: 0.0050% to 0.0150%, V: 0% to 0.300%, Ca: 0% to 0.0100%, Zr: 0% to 0.0100%, Mg: 0% to 0.0100%, and a remainder including Fe and impurities, in which a number density of Mn sulfides having an equivalent circle diameter of more than 5 m is 0 pieces/mm2 to 10 pieces/mm2, and an average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 m to 5.0 μm is 1.0 or more and 10.0 or less.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to steel having a high strength and anexcellent low-temperature toughness after quenching and tempering.

RELATED ART

Recently, in response to changes in energy situation, active effortshave been made across the globe in order to develop new energy sources.In such circumstances, offshore oil fields have drawn attentions assources developed onshore have been depleted, and development usingoil-drilling rigs has been conducted across a broad range of regions,mainly, continental shelves. In particular, recently, the number ofmarine structures represented by offshore oil-drilling rigs that areoperated in the depths of the sea has been rising, and, in order toprevent damage to drilling rigs by large-scale hurricanes, there hasbeen a demand for increasing the strength of chains for mooring drillingrigs. Broken chains lead directly to serious accidents such as thecollapse of rigs. In order to ensure safety which is a vital object, anincrease in both the strength and toughness of chains has been pursued.Specifically, there has been a demand for chains having a tensilestrength of 1,200 MPa or more and a Charpy impact value at −20° C. of 75J/cm² or more.

Such chains are manufactured by cutting a hot rolled steel bar having adiameter of φ50 mm or more to a predetermined length, forming the steelbar to an annular shape, and welding butted end surfaces through flashbutt welding. After flash butt welding, there are cases where a stud ispress-fitted into the center of the annular chain. After that, the chainis quenched and tempered, thereby imparting a high strength and a hightoughness to the chain.

Patent Documents 1 to 6 and the like can be exemplified as inventionexamples of steel for a high strength and high toughness chain. However,all of the documents aim to provide a chain having a tensile strength of800 MPa to 1,000 MPa and do not study a case where the strength of steelis set to 1,200 MPa or more. In recent years, although an additionalincrease in strength has been demanded for chains, it is known that anincrease in the strength of steel generally degrades the toughness ofsteel and thus decreases the impact value of steel. When the strength ofthe steel proposed by the above described documents is set to 1,200 MPaor more, it is not possible to obtain an intended impact value.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] Japanese Unexamined Patent Application. First    Publication No. S58-22361-   [Patent Document 2] Japanese Unexamined Patent Application, First    Publication No. S58-96856-   [Patent Document 3] Japanese Unexamined Patent Application. First    Publication No. S59-159972-   [Patent Document 4] Japanese Unexamined Patent Application, First    Publication No. S59-159969-   [Patent Document 5] Japanese Unexamined Patent Application, First    Publication No. 562-202052-   [Patent Document 6] Japanese Unexamined Patent Application, First    Publication No. S63-203752

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a steel having a highstrength and an excellent low-temperature toughness (particularly,fracture toughness at a low temperature) after quenching and tempering.Specifically, the object of the invention is to provide a steel in whichthe Charpy impact value at −20° C. reaches 75 J/cm² or more, whenquenching and tempering are carried out so that the tensile strengthreaches 1,200 MPa or more.

Means for Solving the Problem

The gist of the present invention is as described below.

(1) According to an aspect of the present invention, there is provided asteel containing, by unit mass %, C: 0.08% to 0.12%, Si: 0.05% to 0.50%,Mn: 1.00% to 3.00%, P: 0.040% or less, S: 0.020% or less, Cr: 1.00% to2.50%, Cu: 0.01% to 0.50%, Ni: 0.75% to 3.20%, Mo: 0.10% to 0.50%, Nb:0.005% to 0.050%, Al: 0.010% to 0.100%, N: 0.0050% to 0.0150%, V: 0% to0.300%, Ca: 0% to 0.0100%, Zr: 0% to 0.0100%, Mg: 0% to 0.0100%, and aremainder including Fe and impurities, in which a number density of Mnsulfides having an equivalent circle diameter of more than 5 μm is 0pieces/mm2 to 10 pieces/mm2, and an average aspect ratio of the Mnsulfides having an equivalent circle diameter of 1.0 μm to 5.0 μm is 1.0or more and 10.0 or less.

(2) The steel according to (1) may contain, by unit mass %, V: 0.010% to0.300%.

(3) The steel according to (1) or (2) may contain, by unit mass %, oneor more selected from the group consisting of Ca: 0.0005% to 0.0100%,Zr: 0.0005% to 0.0100%, and Mg: 0.0005% to 0.0100%.

Effects of the Invention

According to the present invention, it is possible to provide steelhaving a tensile strength of 1,200 MPa or more and a Charpy impact valueat −20° C. of 75 J/cm² or more after quenching and tempering.

EMBODIMENTS OF THE INVENTION

The present inventors have continued a variety of researches in order torealize steel having a high strength and an excellent low-temperaturetoughness, as a result, the present inventors obtained the followingfindings.

(a) In order to impart a tensile strength of 1,200 MPa or more toquenched and tempered steel, a C content in the steel needs to be set to0.08% or more.

(b) When steel contains all of Ni, Mo, and Nb, the low-temperaturetoughness of the steel is improved. The present inventors found that,when steel contains all of Ni, Mo, and Nb, the impact value of the steelis improved. This is considered to be because, when steel contains allof Ni, Mo, and Nb, cementite in the steel which may generally act as anorigin of fracture is refined to a level at which the cementite does notact as a fracture origin. In addition, when steel contains all of Ni,Mo, and Nb, the block size of a martensite becomes small, and thus it isassumed that the ductile-brittle transition temperature of the steel isdecreased and brittle fracture does not easily occur even at a lowtemperature.

(c) The present inventors found that the low-temperature toughness ofsteel can be improved by decreasing the grain size and aspect ratios ofMn sulfides which may act as an origin of fracture.

On the basis of the above described findings, the present inventorsfound the chemical composition, inclusion state, and manufacturingmethod for steel that can be used to manufacture a structural componenthaving a high strength and a high low-temperature toughness,particularly, chains. Hereinafter, a specific aspect of steel accordingto the present embodiment will be described. In addition, although thesteel according to the present embodiment is steel having an effect inwhich the tensile strength reaches 1,200 MPa or more and the Charpyimpact value at −20° C. reaches 75 J/cm² or more after quenching andtempering, the strength and the impact value before quenching andtempering are not particularly limited. Hereinafter, unless particularlyotherwise described, description of mechanical properties such asstrength and toughness relates to the steel according to the presentembodiment after quenching and tempering.

Hereinafter, the reasons for limiting the amounts of individual alloyingelements of the steel according to the present embodiment will bedescribed. The unit “%” of the amounts of the alloying elementsindicates mass %.

C: 0.08% to 0.12%

C is an important element that determines the strength of the steel. Inorder to obtain a tensile strength of 1,200 MPa or more after quenchingand tempering, the lower limit of the C content is set to 0.08%. On theother hand, when the C content is excessive, the strength of the steelis excessively increased, and thus the toughness of the steel isdegraded. In addition, when the C content is excessive, the amount ofcementite which acts as an origin of fracture is increased, and thetoughness of the steel is significantly degraded. Therefore, the upperlimit of the C content is set to 0.12%. The upper limit of the C contentis preferably 0.11%. The lower limit of the C content is preferably0.09%.

Si: 0.05% to 0.50%

Si has an action for ensuring the strength of the steel and also anaction as a deoxidizing agent. When the Si content is less than 0.05%,the deoxidizing action cannot be sufficiently obtained, the number ofnon-metallic inclusions in the steel is increased, and the toughness ofthe steel is degraded. On the other hand, when the Si content is morethan 0.50%, Si causes the degradation in the toughness of the steel.Therefore, the Si content is set to 0.05% to 0.50%. The upper limit ofthe Si content is preferably 0.40%, 0.30%, or 0.20%. The lower limit ofthe Si content is preferably 0.06%, 0.07%, or 0.08%.

Mn: 1.00% to 3.00%

Mn is an essential element for ensuring a desired hardenability. Inorder to ensure sufficient hardenability for setting a tensile strengthof the steel after quenching and tempering to 1,200 MPa or more, thelower limit of the Mn content is set to 1.0%. On the other hand, whenthe Mn content is excessive, the toughness of the steel is degraded, andthus the upper limit of the Mn content is set to 3.00%. The upper limitof the Mn content is preferably 2.90%, 2.80%, or 2.70%. The lower limitof the Mn content is preferably 1.10%, 1.20%, or 1.30%.

P: 0.040% or Less

P is an impurity that is incorporated into the steel during themanufacturing process of the steel. When the P content exceeds 0.040%,the toughness of the steel is degraded more than a permissible limit,and thus the P content is limited to 0.040% or less. The upper limit ofthe P content is preferably 0.030%, 0.025%, or 0.020%. The steelaccording to the present embodiment does not need P, and thus the lowerlimit of the P content is 0%; however, when the capability of a refiningfacility and the like are taken into account, the lower limit of the Pcontent may be set to 0.001%, 0.002%, or 0.003%.

S: 0.020% or Less

S is, similar to P, an impurity that is incorporated into the steelduring the manufacturing process of the steel. When the S contentexceeds 0.020%, S forms a large amount of Mn sulfides in the steel, andthe toughness of the steel is degraded. Therefore, the S content islimited to 0.020% or less. When the S content is 0.020% or less, thenumber density of the Mn sulfides is sufficiently decreased, and thetoughness of the steel is maintained at a high level. The upper limit ofthe S content is preferably 0.015%, 0.012%, or 0.010%. The steelaccording to the present embodiment does not need S, and thus the lowerlimit of the S content is 0%; however, when the capability of a refiningfacility and the like are taken into account, the lower limit of the Scontent may be set to 0.001%, 0.002%, or 0.003%.

Cr: 1.00% to 2.50%

Cr has an action for enhancing the hardenability of the steel. In orderto ensure sufficient hardenability for setting a tensile strength of thesteel after quenching and tempering to 1,200 MPa or more, the lowerlimit of the Cr content is set to 1.00%. On the other hand, when the Crcontent is excessive, the toughness of the steel is degraded. Therefore,the upper limit of the Cr content is set to 2.50%. The upper limit ofthe Cr content is preferably 2.40%, 2.30%, or 2.20%. The lower limit ofthe Cr content is preferably 1.30%, 1.40%, or 1.50%.

Cu: 0.01% to 0.50%

Cu is an effective element for improving the hardenability and corrosionresistance of the steel. In order to ensure sufficient hardenability andcorrosion resistance for setting a tensile strength of the steel afterquenching and tempering to 1,200 MPa or more, the lower limit of the Cucontent is set to 0.01%. On the other hand, when the Cu content isexcessive, the toughness of the steel is degraded. Therefore, the upperlimit of the Cu content is set to 0.50%. The upper limit of the Cucontent is preferably 0.40%, 0.30%, or 0.20%. The lower limit of the Cucontent is preferably 0.02%, 0.03%, or 0.05%.

Ni: 0.75% to 3.20%

Ni is an extremely effective element for improving the toughness of thesteel and an essential element for increasing the toughness of the steelaccording to the present embodiment in which a tensile strength afterquenching and tempering is 1,200 MPa or more. When the Ni content isless than 0.75%, it is difficult to sufficiently exhibit the effects. Onthe other hand, when the Ni content exceeds 3.20%, the effect forimproving toughness is saturated. Therefore, the Ni content is set to0.75% to 3.20%. The upper limit of the Ni content is preferably 3.15%,3.10%, or 3.05%. The lower limit of the Ni content is preferably 0.80%,0.85%, or 0.90%.

Mo: 0.10% to 0.50%

The present inventors found that Mo has an effect for improving thelow-temperature toughness of the steel, when Mo is contained in thesteel together with Ni and Nb. This is considered to be because, whenthe steel contains Mo together with Ni and Nb, cementite in the steelwhich may generally act as an origin of fracture is refined to a levelat which the cementite does not act as a fracture origin. In addition,when the steel contains Mo together with Ni and Nb, the block size of amartensite becomes small, and thus it is assumed that the ductilebrittle transition temperature of the steel is decreased and brittlefracture does not easily occur even at a low temperature. When the Mocontent is less than 0.10%, it is difficult to sufficiently exhibit theeffects. On the other hand, when the Mo content exceeds 0.50%, theeffect for improving toughness is saturated. Therefore, the Mo contentis set to 0.10% to 0.50%. The upper limit of the Mo content ispreferably 0.47%, 0.45%, or 0.42%. The lower limit of the Mo content ispreferably 0.15%, 0.20%, or 0.25%.

Nb: 0.005% to 0.050%

Nb has an effect for improving the low-temperature toughness of thesteel, when Nb is contained in the steel together with Ni and Mo. Thisis considered to be because, when the steel contains Nb together with Niand Mo, cementite in the steel which may generally act as an origin offracture is refined to a level at which the cementite does not act as afracture origin. In addition, when the steel contains Nb together withNi and Mo, the block size of a martensite becomes small, and thus it isassumed that the ductile brittle transition temperature of the steel isdecreased and brittle fracture does not easily occur even at a lowtemperature. When the Nb content is less than 0.005%, it is difficult tosufficiently exhibit the effects. On the other hand, when the Nb contentexceeds 0.050%, the effect for improving toughness is saturated.Therefore, the Nb content is set to 0.005% to 0.050%. The upper limit ofthe Nb content is preferably 0.045%, 0.040%, or 0.035%. The lower limitof the Nb content is preferably 0.007%, 0.010%, or 0.015%.

Al: 0.010% to 0.100%

In addition to a deoxidizing action, A1 has an action for adjusting thecrystal grain size of a metallographic structure and miniaturizing themetallographic structure when A1 is precipitated as AlN. When the A1content is less than 0.010%, it is not possible to obtain a sufficientminiaturizing effect, and thus the toughness of the steel is degraded.On the other hand, when the A1 content in the steel exceeds 0.100%, theamount of AlN precipitated is saturated, the number of alumina basednon-metallic inclusions in the steel is increased, and the toughness ofthe steel is degraded. Therefore, the A1 content is set to 0.010% to0.100%. The upper limit of the A1 content is preferably 0.090%, 0.070%,or 0.050%. The lower limit of the A1 content is preferably 0.012%,0.015%, or 0.018%.

N: 0.0050% to 0.0150%

N has an action for precipitating AlN, which is effective for adjustingthe crystal grain size of the metallographic structure, by bonding toA1. When the N content is less than 0.0050%, this action is notsufficiently exhibited. On the other hand, when the N content in thesteel exceeds 0.0150%, the number of solute N is increased, and thetoughness of the steel is degraded. Therefore, the N content is set to0.0050% to 0.0150%. The upper limit of the N content is preferably0.0140%, 0.0130%, or 0.0120%. The lower limit of the N content ispreferably 0.0055%, 0.0060%, or 0.0065%.

V: 0% to 0.300%

The steel according to the present embodiment does not need V.Therefore, the lower limit of the V content is 0%. However, V has anaction for adjusting the crystal grain size of the metallographicstructure and miniaturizing the metallographic structure when V isprecipitated as VN. Therefore, as an optional element, the steel maycontain 0.010% or more, 0.020% or more, or 0.030% or more of V. On theother hand, when the V content in the steel exceeds 0.300%, coarse VNremains in the steel after heating for quenching, and this coarse VNdegrades the toughness of the steel after quenching and tempering.Therefore, the V content is set to 0.300% or less. The upper limit ofthe V content is preferably 0.250% or less, 0.200%, or 0.150%.

One or more selected from the group consisting of Ca: 0% to 0.0100%, Zr:0% to 0.0100% or less, and Mg: 0% to 0.0100%

The steel according to the present embodiment does not need Ca, Zr, andMg. Therefore, the lower limit of the V content is 0%. However, all ofCa. Zr, and Mg have an effect for forming an oxide, acting as acrystallization nucleus of MnS, and uniformly and finely dispersing MnSso as to improve the impact value of the steel. Therefore, as anoptional element, the steel may contain 0.0005% or more, 0.0010% ormore, or 0.0015% or more of Ca, may contain 0.0005% or more, 0.0010% ormore, or 0.0015% or more of Zr, and may contain 0.0005% or more, 0.0010%or more, or 0.0015% or more of Mg. On the other hand, when each of theCa content, the Zr content, and the Mg content exceeds 0.0100%, anexcess amount of a hard inclusion such as an oxide and a sulfide isgenerated, and the toughness of the steel is degraded. Therefore, theupper limits of each of the Ca content, the Zr content, and the Mgcontent is set to 0.0100% or less. The upper limit of the Ca content ispreferably 0.0090%, 0.0070%, or 0.0050%, the upper limit of the Zrcontent is preferably 0.0090%, 0.0070%, or 0.0050%, and the upper limitof the Mg content is preferably 0.0090%, 0.0070%, or 0.0050%.

Remainder: Fe and Impurities

The remainder of the chemical composition of the steel according to thepresent embodiment consists of Fe and impurities. The impurities referto elements which are incorporated by a raw material such as an ore or ascrap, or a variety of causes in the manufacturing process during theindustrial manufacturing of the steel, and the impurities are permittedto an extent in which the steel according to the present embodiment isnot adversely influenced.

Next, the reason for limiting the inclusion state of the steel accordingto the present embodiment will be described.

Number density of Mn sulfides having an equivalent circle diameter ofmore than 5 μm is 0 pieces/mm² to 10 pieces/mm²

A Mn sulfide having an equivalent circle diameter of more than 5 μm(hereinafter, referred to as a “coarse Mn sulfide”) significantlydegrades the low-temperature toughness of the steel, and thus the numberdensity of the coarse Mn sulfides is preferably set to substantially 0pieces/mm². Therefore, the lower limit of the number density of thecoarse Mn sulfides is 0 pieces/mm². However, when the number density is10 pieces/mm² or less, the low-temperature toughness is not seriouslyimpaired. Therefore, the upper limit of the number density of the coarseMn sulfides is set to 10 pieces/mm². The upper limit of the numberdensity of the coarse Mn sulfides is preferably 9 pieces/mm², 8pieces/mm², or 7 pieces/mm².

Average aspect ratio of the Mn sulfides having an equivalent circlediameter of 1.0 μm to 5.0 μm is 1.0 or more and 10.0 or less

A Mn sulfide having an equivalent circle diameter of 1.0 μm to 5.0 μm(hereinafter, referred to as a “fine Mn sulfide”) has a smaller adverseinfluence on the toughness of the steel than the coarse Mn sulfide.However, a fine Mn sulfide, in which the aspect ratio of the Mn sulfidethat can be calculated by dividing the major axis of the Mn sulfide bythe minor axis of the Mn sulfide is excessively large, may act as anorigin of fracture and degrade the toughness of the steel, similar tothe coarse Mn sulfide. The present inventors found that, when theaverage aspect ratio of the fine Mn sulfides is set to 10.0 or less, itis possible to make the fine Mn sulfides almost harmless. A preferredupper limit of the average aspect ratio of the fine Mn sulfides is 9.0,7.5, or 6.0. When the major axis and the minor axis of the fine Mnsulfide are equal to each other, the aspect ratio of the fine Mn sulfidereaches 1.0, and thus the lower limit of the average aspect ratio of thefine Mn sulfides is set to 1.0.

The fine dispersion of Mn sulfides which may act as origins of fractureand a decrease in the aspect ratios thereof are extremely effective forimproving the low-temperature toughness of the steel. In addition, sincethe state of the Mn sulfides does not change before and after quenchingand tempering that are carried out under ordinary conditions, the stateof the Mn sulfides is maintained even after quenching and tempering aslong as the state of the Mn sulfides is controlled as described abovebefore quenching and tempering, and the above described effects can beobtained.

In addition, in the steel according to the present embodiment, there isno need for limiting the number density of the fine Mn sulfides.Although there is concern that an extremely large amount of the fine Mnsulfides impairs the toughness of the steel, the number density of thefine Mn sulfides does not increase to an extent in which the toughnessof the steel is impaired, as long as the S content is in the abovedescribed range. Furthermore, in the steel according to the presentembodiment, a Mn sulfide having an equivalent circle diameter of lessthan 1.0 μm (hereinafter, referred to as an “ultrafine Mn sulfide”) doesnot act as an origin of fracture, and thus the aspect ratio and numberdensity of the ultrafine Mn sulfide are not particularly specified.Furthermore, in the steel according to the present embodiment, the Mnsulfides (the coarse Mn sulfides and the fine Mn sulfides) are almostuniformly dispersed, and thus a place where the state of the Mn sulfideis specified is not particularly limited.

A method for specifying the state of the Mn sulfide is as describedbelow. First, a cross section of the steel is mirror-polished, and thenoptical microscopic photographs are captured at 10 or more random placeson the cross section at a magnification of 1,000 times. The tenphotographs obtained in the above described manner are processed usingimage analysis software, for example, Luzex (registered trademark) orthe like, whereby the state of Mn sulfides in the steel, that is, thenumber density of the coarse Mn sulfides and the average aspect ratio ofthe fine Mn sulfides can be obtained. In the steel according to thepresent embodiment, Mn sulfides are elongated in a processing direction.For example, when the steel is hot rolled, the Mn sulfides are elongatedin a hot rolling direction. Therefore, the cross section where theoptical microscopic photographs are captured needs to be formed parallelto the processing direction (for example, the hot rolling direction). Onthe other hand, since the Mn sulfides are almost uniformly dispersed inthe steel according to the present embodiment, and a place where theoptical microscopic photographs are captured is not particularlyspecified.

Next, a method for manufacturing the steel according to the presentembodiment will be described.

The method for manufacturing the steel according to the presentembodiment includes a process of continuously casting molten steelhaving the chemical composition of the steel according to the presentembodiment so as to obtain a slab and a process of soaking the slabtwice or more. The conditions for continuously casting the molten steelare not particularly limited. In the process of soaking the slab,firstly, the slab is heated up to a temperature range of 1,300° C. to1,350° C., and then, the temperature of the slab is held in thistemperature range for 300 seconds to 18,000 seconds, and furthermore,the slab is cooled to 900° C. or lower. In addition, the soaking iscarried out twice or more.

(Soaking)

The soaking is carried out in order to finely disperse Mn sulfidesincluded in the slab. During the continuous casting, coarse Mn sulfidesare crystallized in the slab. When the slab is heated up to atemperature range of 1,300° C. to 1,350° C. and then held in thistemperature range for 300 seconds to 18,000 seconds, the coarse Mnsulfides are solutionized, and the Mn sulfides are precipitated when theslab is cooled to 900° C. or lower. The Mn sulfides are refined bysolutionizing and precipitation.

When the holding temperature of the slab is lower than 1,300° C. andwhen the holding time of the slab at the temperature is shorter than 300seconds, the Mn sulfides are not sufficiently solutionized. In addition,when the soaking is carried out only once, the Mn sulfides are notsufficiently refined. In order to set the dispersion state of the Mnsulfides in the steel in the above described range, the soaking underthe above described conditions needs to be carried out twice or more.When the cooling stop temperature of the slab is set to higher than 900°C. and the subsequent soaking is initiated, the Mn sulfides are notprecipitated during cooling, and thus the refinement of the Mn sulfidesbecomes insufficient.

Meanwhile, when the heating temperature of the slab is higher than1,350° C., the ductility of the slab is degraded, and a problem ofcracking is caused. In addition, when the heating time of the slab islonger than 18,000 seconds, it is not preferable in consideration of theeconomic efficiency.

On the slab in which the Mn sulfides are sufficiently refined by theabove described treatment, it is possible to carry out an optionalprocessing and an optional heat treatment afterwards. For example,blooming and hot rolling are performed on this slab so as to produce asteel bar, and a chain processing is performed on this steel bar,whereby a chain can be obtained. In addition, during or after the chainprocessing, it is possible to quench and temper the chain. Since the Mnsulfides included in the slab obtained using the above described methodare sufficiently refined, and it is assumed that the fine Mn sulfidesincluded in the slab are not outside the specification range describedabove due to blooming, hot rolling, the chain processing, and quenchingand tempering that are carried out under ordinary conditions.

Even when the steel according to the present embodiment is quenched andtempered so that the tensile strength reaches 1,200 MPa or more, theCharpy impact value at −20° C. can be maintained at 75 J/cm² or more.Therefore, the steel according to the present embodiment is particularlypreferably used as steel for quenching and tempering.

For example, when a quenching treatment in which steel is heated to 900°C., held for 30 minutes, and then cooled with water is performed on thesteel according to the present embodiment, and furthermore, a temperingtreatment in which the steel is heated to 135° C. and held for 30minutes is performed on the steel, steel having a tensile strength of1,200 MPa or more and a Charpy impact value at −20° C. of 75 J/cm² ormore is obtained. In the steel according to the present embodiment onwhich the heat treatment under the above described quenching andtempering conditions is performed, the number density of Mn sulfideshaving an equivalent circle diameter of more than 5 μm is 0 pieces/mm²to 10 pieces/mm², the average aspect ratio of the Mn sulfides having anequivalent circle diameter of 1.0 μm to 5.0 μm is 1.0 or more and 10.0or less, the average grain size of cementite is 0.05 μm or less, and theaverage size of martensite blocks is 5.5 μm or less. The steel accordingto the present embodiment contains 0.08% or more of C and thus has atensile strength of 1,200 MPa or more, when the heat treatment under theabove described quenching and tempering conditions is performed.Generally, the low-temperature toughness (particularly, low-temperaturetoughness) is impaired when the tensile strength of the steel is 1,200MPa or more. However, the steel according to the present embodimentcontains 0.75% to 3.20% of Ni, 0.10% to 0.50% of Mo, and 0.005% to0.050% of Nb, and thus, when the heat treatment under the abovedescribed quenching and tempering conditions is performed, martensiteblocks and cementite are sufficiently refined, and the steel has a highlow-temperature toughness. In addition, in the steel according to thepresent embodiment on which the heat treatment under the above describedquenching and tempering conditions is performed, similar to the steelaccording to the present embodiment before quenching and tempering, thenumber density of Mn sulfides having an equivalent circle diameter ofmore than 5 μm is 0 pieces/mm² to 10 pieces/mm², and the average aspectratio of the Mn sulfides having an equivalent circle diameter of 1.0 μmto 5.0 μm is 1.0 or more and 10.0 or less, and thus the steel has a highlow-temperature toughness.

Meanwhile, quenching and tempering under the above described conditionsare simply an example of the use of the steel according to the presentembodiment. According to the purposes, a heat treatment under optionalconditions can be performed on the steel according to the presentembodiment. In addition, the properties of the steel according to thepresent embodiment on which the heat treatment is performed on the basisof an example of the above described quenching and tempering conditionsdo not limit the technical scope of the steel according to the presentembodiment. The object of the steel according to the present embodimentis to obtain a Charpy impact value at −20° C. of 75 J/cm² or more aftera heat treatment is carried out so that the tensile strength reaches1,200 MPa. As described above, in order to achieve this object, it isnecessary to control the chemical composition and the Mn sulfide statebefore the heat treatment. However, other constitutions, for example,the states of martensite and cementite before the heat treatment and thelike do not need to be controlled in order to achieve the object of thesteel according to the present embodiment.

In addition, quenching and tempering under ordinary conditions do nothave any influences on the Mn sulfide state. Therefore, when the Mnsulfide state in quenched and tempered steel is in the specificationrange described above, it is assumed that the Mn sulfide state beforequenching and tempering in the steel is also in the specification rangedescribed above.

The steel according to the present embodiment is capable of exhibitingparticularly excellent effects, when the steel according to the presentembodiment is used as a material for chains for mooring offshoreoil-drilling rigs which needs to have a high tensile strength and a highlow-temperature toughness.

EXAMPLES

Hereinafter, the present invention will be described in detail usingexamples. Meanwhile, these examples are intended to describe thetechnical meaning and effects of the present invention and do not limitthe scope of the present invention.

Example 1

Steel A having a chemical composition shown in Table 1 was continuouslycast so as to obtain a slab, then, a soaking was performed on the slabonce or more, and furthermore, a blooming was performed on the slab,thereby obtaining a 162 mm×162 mm rolled material. The conditions forthe soaking and the number of times of the soaking are shown in Table 2.After that, hot rolling was performed on the rolled material, therebyproducing a round steel bar having a diameter of 86 mm. Next, aquenching treatment in which the round steel bar was cut, heated to 900°C., held for 30 minutes, and furthermore, cooled with water was carriedout, and then a tempering treatment in which the round steel bar washeated to 135° C. and held for 30 minutes was carried out, therebyobtaining round steel bars Nos. A1 to A5. These quenching conditions andtempering conditions are the same as heat treatment conditions that arerecommended for the production of chains using the present inventionsteel.

Three JIS No. 14A tensile test pieces and four JIS No. 4 V-notch Charpyimpact test pieces were produced from a ¼D portion (a region at a depthof approximately ¼ of a diameter D of the round steel bar from thesurface of the round steel bar) of a C cross section of each of thequenched and tempered round steel bars Nos. A1 to A5. A tensile test wascarried out at normal temperature and a rate of 20 mm/min according toJIS Z 2241. A Charpy impact test was carried out at −20° C. according toJIS Z 2242.

Furthermore, a 10 mm×10 mm sample was cut out from the ¼D portion of theC cross section of each of the quenched and tempered round steel barsNos. A1 to A5, and the metallographic structure of the steel and thestate of inclusions were observed on a cross section parallel to arolling direction. In order to observe Mn sulfides present in the steel,the cross section was mirror-polished, then, 10 metallographicphotographs were captured using an optical microscope at a magnificationof 1,000 times, and the equivalent circle diameters and aspect ratios ofMn sulfides included in the photographs were obtained by means of animage analysis (Luzex (registered trademark)). In addition, in order toobserve cementite present in the steel, the cross section was corrodedwith a nital etching solution, five metallographic photographs werecaptured using a scanning electron microscope at a magnification of5,000 times, and the average grain size of the cementite included in thephotographs was obtained by means of an image analysis (Luzex(registered trademark)). Furthermore, a crystal orientation analysis wascarried out on the sample using an electron backscatter diffractionpattern, and the area-weighted average equivalent circle diameter ofcrystal grains surrounded by high angle grain boundaries which had anorientation difference angle of 15 degrees, which was obtained from theabove described analysis, was considered as the average grain size ofmartensite blocks.

The results of the above described experiments are shown in Table 1 andTable 2. Table 1 shows the chemical compositions of the steel A (thatis, the chemical compositions of the steels No. A1 to No. A5). Table 2shows soaking conditions and the number of times of soaking during themanufacturing of the steels No. A1 to No. A5, and the average aspectratios of Mn sulfides having an equivalent circle diameter of 1.0 μm to5.0 μm, the number densities of Mn sulfides having an equivalent circlediameter of more than 5.0 μm, the tensile strengths, the impact values,the average grain sizes of the cementite, and the average sizes of themartensite blocks in the steels No. A1 to No. A5 which were quenched andtempered under the above described conditions. In Table 2, valuesoutside the specification ranges of the present invention areunderlined. Meanwhile, quenching and tempering under the above describedconditions do not have any influences on the state of the Mn sulfides,and thus the states of the Mn sulfides in the quenched and temperedsteels No. A1 to No. A5 disclosed in Table 2 are the same as those ofthe steels No. A1 to No. A5 before quenching and tempering.

TABLE 1 Chemical composition (mass %) Steel type C Si Mn P S Cr Cu Ni MoNb Al N A 0.10 0.21 1.81 0.006 0.007 1.72 0.09 1.52 0.33 0.015 0.0230.0085

TABLE 2 Number density Average aspect of Mn sulfides Average Soakingconditions ratio of Mn sulfides having equivalent grain size AverageHeating Number of having equivalent circle diameter of Tensile Impact ofsize of temperature Holding times of circle diameter of more than 5.0 μmstrength value cementite martensite No. (° C.) time (sec.) soaking 1.0μm to 5.0 μm (pieces/mm²) (MPa) (J/cm²) (μm) blocks (μm) ClassificationA1 1300 7200 3  1.3  0.0 1308 171 0.03 3.7 Invention A2 1350 7200 2  4.5 1.1 1321 154 0.04 3.8 Example A3 1290 7200 2 10.8 10.2 1324 68 0.03 3.8Comparative A4 1350 290 2 10.5 10.8 1311 54 0.03 3.8 Example A5 13007200 1 10.3 11.1 1284 65 0.03 4.1

As shown in Table 1 and Table 2, in the steels No. A1 and No. A2 whichwere the present invention example, the chemical compositions and themanufacturing conditions were appropriate, and thus the form of the Mnsulfides was in the specification range of the present invention.Therefore, the steels No. A1 and A2 had a tensile strength of 1,200 MPaor more and a Charpy impact value at −20° C. of 75 J/cm² or more afterquenching and tempering. In contrast, in the steels No. A3 to A5 whichwere comparative examples, the manufacturing conditions were notappropriate, and thus the Mn sulfides coarsened or the aspect ratios ofthe Mn sulfides increased, and the low-temperature toughness afterquenching and tempering was insufficient.

Example 2

Each of steel B to AH having a chemical composition shown in Table 3 wascontinuously cast so as to obtain a slab, then, a soaking in which theholding temperature was 1,300° C. and the holding time was 7,200 secondswas performed on the slab twice, and furthermore, blooming was performedon the slab, thereby obtaining a 162 mm×162 mm rolled material. Afterthat, hot rolling was performed on the rolled material, therebyproducing a round steel bar having a diameter of 86 mm. Next, aquenching treatment in which the round steel bar was cut, heated to 900°C., held for 30 minutes, and furthermore, cooled with water was carriedout, and then a tempering treatment in which the round steel bar washeated to 135° C. and held for 30 minutes was carried out, therebyobtaining round steel bars Nos. B to AH. These quenching conditions andtempering conditions are the same as heat treatment conditions that arerecommended for the production of chains using the present inventionsteel.

Three JIS No. 14A tensile test pieces and four JIS No. 4 V-notch Charpyimpact test pieces were produced from a ¼D portion of a C cross sectionof each of the quenched and tempered round steel bars Nos. B to AH. Atensile test was carried out at normal temperature and a rate of 20mm/min according to JIS Z 2241. A Charpy impact test was carried out at−20° C. according to JIS Z 2242.

Furthermore, a 10 mm×10 mm sample was cut out from the ¼D portion of theC cross section of each of the quenched and tempered round steel barsNos. B to AH, and the metallographic structure of the steel and thestate of inclusions were observed on a cross section parallel to arolling direction. In order to observe Mn sulfides present in the steel,the cross section was mirror-polished, then, 10 metallographicphotographs were captured using an optical microscope at a magnificationof 1,000 times, and the equivalent circle diameters and aspect ratios ofMn sulfides included in the photographs were obtained by means of animage analysis (Luzex (registered trademark)). In addition, in order toobserve cementite present in the steel, the cross section was corrodedwith a nital etching solution, five metallographic photographs werecaptured using a scanning electron microscope at a magnification of5,000 times, and the average grain size of the cementite included in thephotographs was obtained by means of an image analysis (Luzex(registered trademark)). Furthermore, a crystal orientation analysis wascarried out on the sample using a backscattered electron beamdiffraction pattern, and the area-weighted average equivalent circlediameter of crystal grains surrounded by high angle grain boundarieswhich had an orientation difference angle of 15 degrees, which wasobtained from the above described analysis, was considered as theaverage grain size of martensite blocks.

The results of the above described experiments are shown in Table 3 andTable 4. Table 3 shows the chemical compositions of the steels Nos. B toAH. Table 4 shows the aspect ratios of Mn sulfides having an equivalentcircle diameter of 1.0 μm to 5.0 μm, the number densities of Mn sulfideshaving an equivalent circle diameter of more than 5.0 μm, the tensilestrengths, the impact values, the average grain sizes of the cementite,and the average sizes of the martensite blocks in the steels Nos. B toAH which were quenched and tempered under the above describedconditions. In Table 3 and Table 4, values outside the specificationranges of the present invention are underlined. Meanwhile, quenching andtempering under the above described conditions do not have anyinfluences on the state of the Mn sulfides, and thus the states of theMn sulfides in the quenched and tempered steels Nos. B to AH disclosedin Table 4 are the same as those of the steels Nos. B to AH beforequenching and tempering.

TABLE 3 Steel Chemical composition (mass %) type C Si Mn P S Cr Cu Ni MoNb Al N V Others Classification B 0.10 0.07 2.07 0.009 0.007 1.82 0.070.75 0.36 0.019 0.023 0.0059 — — Invention C 0.08 0.09 1.78 0.008 0.0031.81 0.10 2.52 0.35 0.013 0.024 0.0093 0.038 — Example D 0.12 0.10 1.330.007 0.007 1.78 0.08 3.18 0.35 0.015 0.017 0.0113 — — E 0.10 0.11 1.660.008 0.005 1.56 0.09 2.45 0.33 0.022 0.018 0.0075 0.031 — F 0.10 0.051.37 0.005 0.004 1.57 0.10 2.90 0.37 0.020 0.016 0.0119 — Ca: 0.003% G0.10 0.09 1.79 0.008 0.004 1.66 0.08 2.50 0.40 0.021 0.020 0.0109 0.042Mg: 0.005% H 0.10 0.50 1.92 0.008 0.006 1.56 0.10 2.62 0.33 0.021 0.0240.0080 — Zr: 0.009% I 0.10 0.10 1.01 0.007 0.005 1.57 0.10 2.42 0.340.019 0.017 0.0054 — — J 0.10 0.09 2.98 0.006 0.004 1.03 0.09 2.74 0.350.015 0.021 0.0113 — — K 0.10 0.09 1.51 0.005 0.005 2.47 0.11 2.77 0.350.016 0.021 0.0087 — — L 0.09 0.07 1.35 0.007 0.005 1.58 0.07 2.46 0.110.015 0.017 0.0118 0.053 — M 0.09 0.10 1.40 0.007 0.005 1.54 0.10 2.620.50 0.022 0.018 0.0077 — — N 0.10 0.08 1.34 0.008 0.006 1.55 0.08 2.640.31 0.005 0.021 0.0112 — — O 0.09 0.09 1.41 0.008 0.008 1.61 0.09 2.740.32 0.048 0.019 0.0091 — — P 0.09 0.10 1.62 0.009 0.007 1.63 0.08 2.810.35 0.015 0.011 0.0073 — — Q 0.10 0.10 1.58 0.006 0.007 1.54 0.10 2.680.33 0.016 0.098 0.0077 — — R 0.10 0.09 1.63 0.039 0.006 1.55 0.11 2.520.32 0.021 0.022 0.0064 0.028 Ca: 0.005% S 0.09 0.09 1.58 0.007 0.0191.62 0.09 2.37 0.32 0.019 0.020 0.0084 — — T 0.10 0.10 1.52 0.008 0.0071.54 0.02 2.41 0.37 0.018 0.019 0.0093 — — U 0.10 0.08 1.42 0.008 0.0081.42 0.48 2.57 0.35 0.022 0.022 0.0078 — — V 0.10 0.10 1.41 0.006 0.0051.71 0.11 2.82 — — 0.022 0.0062 — — Comparative W 0.09 0.10 1.61 0.0050.004 1.82 0.08 2.98 0.32 — 0.024 0.0096 — — Example X 0.09 0.07 1.670.007 0.004 1.47 0.08 2.73 — 0.022 0.021 0.0117 — — Y 0.09 0.08 1.510.006 0.006 1.64 0.09 0.73 0.45 0.019 0.021 0.0080 — — Z 0.10 0.08 1.370.006 0.005 1.72 0.08 2.47 0.08 0.014 0.019 0.0081 0.032 Ca: 0.005% AA0.10 0.09 0.43 0.008 0.006 1.64 0.09 2.72 0.34 0.003 0.022 0.0077 — Ma:0.007% AB 0.07 0.10 1.91 0.006 0.005 1.73 0.10 2.65 0.30 0.023 0.0250.0065 0.045 — AC 0.13 0.09 1.46 0.007 0.004 1.81 0.07 2.52 0.30 0.0170.024 0.0055 — — AD 0.09 0.51 1.74 0.006 0.007 1.70 0.09 2.55 0.43 0.0190.022 0.0083 — — AE 0.09 0.09 3.20 0.008 0.004 1.68 0.08 2.81 0.40 0.0150.023 0.0057 — — AF 0.09 0.09 1.32 0.009 0.006 0.97 0.09 3.08 0.33 0.0220.018 0.0063 — — AG 0.10 0.10 1.37 0.008 0.021 1.63 0.09 2.74 0.33 0.0190.020 0.0094 0.024 Zr: 0.009% AH 0.09 0.09 2.08 0.007 0.004 1.84 0.082.90 0.38 0.023 0.023 0.0154 — —

TABLE 4 Average aspect ratio of Number density of Mn Mn sulfides havingsulfides having equivalent circle equivalent circle diameter TensileImpact Average grain Average size of Steel diameter of 1.0 μm to of morethan 5 μm strength value size of cementite martensite blocks type 5.0 μm(pieces/mm²) (MPa) (J/cm²) (μm) (μm) Classification B 4.1 2.1 1278 1560.03 3.2 Invention C 3.9 0.0 1222 150 0.04 3.9 Example D 4.5 1.3 1370170 0.04 3.8 E 4.2 0.7 1315 181 0.04 4.4 F 3.7 0.0 1301 167 0.04 4.8 G4.3 0.4 1307 153 0.04 3.6 H 4.1 1.1 1290 127 0.04 4.4 I 4.2 1.6 1304 1310.04 3.4 J 3.8 0.6 1318 187 0.03 3.7 K 4.1 0.8 1285 170 0.04 4.9 L 4.31.3 1285 164 0.04 4.1 M 4.5 1.7 1296 128 0.03 3.1 N 3.9 1.1 1311 1270.03 3.8 O 4.3 0.3 1308 134 0.04 3.7 P 4.4 0.7 1297 130 0.03 3.6 Q 4.11.2 1287 136 0.03 3.8 R 3.8 1.8 1303 129 0.04 3.9 S 4.2 1.2 1298 1320.03 3.6 T 4.0 2.0 1311 138 0.04 3.7 U 4.3 1.4 1318 128 0.04 3.7 V 4.22.2 1279 49 0.07 10.2 Comparative W 4.0 0.3 1296 70 0.06 8.7 Example X3.7 0.0 1294 57 0.06 9.1 Y 4.4 2.1 1267 41 0.06 9.5 Z 4.1 0.8 1302 630.07 9.8 AA 4.3 1.2 1297 54 0.06 9.9 AB 4.2 0.7 1193 162 0.04 3.6 AC 4.10.6 1415 68 0.04 3.3 AD 4.5 1.4 1282 58 0.04 5.1 AE 4.3 2.1 1294 46 0.044.7 AF 4.3 0.2 1266 71 0.04 5.4 AG 4.1 5.6 1309 52 0.03 4.3 AH 4.0 0.01282 67 0.03 3.8

As shown in Table 3 and Table 4, in all of the steels Nos. B to U whichwere the present invention example, the chemical compositions and thestates of the Mn sulfides were in the specification range of the presentinvention. Therefore, the steels Nos. B to U had a tensile strength of1,200 MPa or more and a Charpy impact value at −20° C. of 75 J/cm² ormore after quenching and tempering.

In contrast, in the steels Nos. V, W, X, Y, Z, and AA which werecomparative examples, the amount of one or more of Mo, Nb, and Ni wasinsufficient or one or more of Mo, Nb, and Ni was not included, andthus, after quenching and tempering, cementite which acted as an originof fracture became coarse, furthermore, the average size of martensiteblocks became coarse, and the low-temperature toughness wasinsufficient.

In the steel No. AB which was a comparative example, the C content wasinsufficient, and thus a necessary tensile strength could not beobtained after quenching and tempering. Meanwhile, in the steel No. ACwhich was a comparative example, the C content was excessive, and thusthe strength became excessively high, and the low-temperature toughnessafter quenching and tempering was insufficient.

In the steel No. AD which was a comparative example, the Si content wasexcessive, and in the steel No. AE, the Mn content was excessive. Theexcess Si or Mn degraded the toughness of the steel, and thus thelow-temperature toughness of the steels No. AD and No. AE afterquenching and tempering was insufficient.

In the steel No. AF which was a comparative example, the Cr content wasinsufficient, and thus sufficient hardenability could not be obtained,and the low-temperature toughness after quenching and tempering wasinsufficient.

In the steel No. AG which was a comparative example, the S content wasexcessive, and thus an excess amount of Mn sulfides were formed, and thelow-temperature toughness after quenching and tempering wasinsufficient. In the steel No. AH which was a comparative example, the Ncontent was excessive, and thus the amount of solute N became excessive,and the low-temperature toughness after quenching and tempering wasinsufficient.

1. A steel comprising, by unit mass %, C: 0.08% to 0.12%; Si: 0.05% to0.50%; Mn: 1.00% to 3.00%; P: 0.040% or less; S: 0.020% or less; Cr:1.00% to 2.50%; Cu: 0.01% to 0.50%; Ni: 0.75% to 3.20%; Mo: 0.10% to0.50%; Nb: 0.005% to 0.050%; Al: 0.010% to 0.100%; N: 0.0050% to0.0150%; V: 0% to 0.300%; Ca: 0% to 0.0100%; Zr: 0% to 0.0100%; Mg: 0%to 0.0100%; and a remainder including Fe and impurities, wherein anumber density of Mn sulfides having an equivalent circle diameter ofmore than 5 μm is 0 pieces/mm² to 10 pieces/mm², and an average aspectratio of the Mn sulfides having an equivalent circle diameter of 1.0 μmto 5.0 μm is 1.0 or more and 10.0 or less.
 2. The steel according toclaim 1 comprising, by unit mass %, V: 0.010% to 0.300%.
 3. The steelaccording to claim 1 comprising, by unit mass %, one or more selectedfrom the group consisting of Ca: 0.0005% to 0.0100%; Zr: 0.0005% to0.0100%; and Mg: 0.0005% to 0.0100%.
 4. The steel according to claim 2comprising, by unit mass %, one or more selected from the groupconsisting of Ca: 0.0005% to 0.0100%; Zr: 0.0005% to 0.0100%; and Mg:0.0005% to 0.0100%.